Offset printing machines customarily have an inking system in which ink is supplied from an ink trough or ink supply furnishing element, which may include a doctor blade, or the like, through a plurality of rollers, some of which are axially oscillating, to the plate cylinder of the printing machine. Some rotary offset printing machines have a forme roller which is in engagement with the plate cylinder. The forme roller may have the same diameter as the plate cylinder or, in any event, is of larger diameter than the remaining ink transfer or oscillating or milling rollers within the ink roller train (see German Published Patent Application DE-AS No. 23 23 025). The ink film supplied by a forme roller of a diameter which is equal to or at least closely approaches that of the plate cylinder can be made to be essentially uniform, and the film thickness can be accurately adjusted. Such apparatus is eminently suitable for high-quality printing in which ghosts and streaking are effectively avoided. The mutual engagement surfaces between the plate cylinder and the forme roller, and hence the engaging run-off of the two cylinders against each other, is 1:1. This system, however, has the difficulty that the damping fluid, typically water, which is supplied to the plate cylinder, for example by the forme roller, cannot be readily removed, so that water spots or pits may result in the final print-out.
It has previously been proposed to prevent penetration of damping liquid into the ink train by applying an air blast to a roller within the ink train so that any damping liquid which might be transferred into the ink furnishing system is evaporated--see German Utility Model 1 904 531. This arrangement is, essentially, a drying system, and utilizes an axially oscillating milling roller in the ink train which is subjected to the air blast.
It has also been proposed to supply ink to the plate cylinder by using two parallel ink trains--see, for example, German Published Patent Application DE-AS No. 20 12 232. The two ink trains are in continuous rolling engagement. No provision is made for removal of excess damping liquid which may be carried along by the plate cylinder.
Current rotary offset printing presses must have a high degree of versatility, that is, the printing machine itself should be so constructed that it is capable of printing on various types of substrates. Due to the different absorbency of ink and wetting liquid of paper, different requirements are placed on the printing system when, for example, printing is to be effected on newsprint or on highly calendared paper, or, in an extreme case, on metal foil which is not absorbent at all.